The nELBE beam scanner

At nELBE a beam scanner is available to scan the spacial profile of the neutron beam. It consists of two plastic scintillator strips that are 1 m long/42 mm thick/11 mm wide. Each strip is equiped with two Hamamatsu R2059-01 photomultiplier tubes (PMT), one at each end. Both detectors can be moved by means of Berger-Lahr stepping motors. One detector moves in vertical the other one in horizontal direction.

How to move the detectors

The movement of the scanners is controled by means of a LabView program installed on PC FWK21.

* numbers in brackets corespond to the arrows shown in Figures 1 and 2

Figure 1: Illustration of the sequence to start the motor control

Figure 2: Illustration of the sequence to start the motor control - Confirm error message

Table 1: Assignment of the PMTs of the two scanner detectors

PMT

H-up

H-down

V-left*

V-right*

material

EJ-230

EJ-200

size

1000x42x11 mm3

1000x42x11 mm3

movement direction

horizontal

vertical

COM port

5

4

motor address

1

1

zero position

right*

down

maximum position

865 mm

865 mm

beam position

437 mm

437 mm

voltage divider no.

55

10

1

26

HV channel

05/4

05/5

05/6

05/7

HV cable

T1

T2

T3

T4

patch field channel

19

20

29

30

patch cable (12 m)

1

2

3

4

* in beam direction

Table 2: Recommended voltage combinations

PMT

H-up

H-down

V-left

V-right

HV channel

05/4

05/5

05/6

05/7

voltage / V

3000

2630

3000

2650

2800

2480

2800

2500

2600

2330

2600

2350

2400

2150

2400

2200

2200

2000

2200

2050

2000

1700

2000

1800

1800

1500

1800

1600

The different buttons and text fields have the following meanings:

Button

Meaning

Comment

Port

COM port of the USB to RS485 Converter

Should be changed only before starting the program

motor adress

Address of the motor

Can be changed by an internal rotary switch

desired speed

Velocity of the scanner used for the next movement

Can be choosen in multiples of 12.5 mm/min Default value is 125 mm/min Maximum value is 400 mm/min

distance

Distance to go during the next movement

position (absolute)

Final position of the next movement

power on/off motor

Button to switch on the motor

Has to be pressed before any movement can be done

<--

Move motor over user defined distance into positiv direction

Pos

Move motor to user defined position

-->

Move motor over user defined distance into negative direction

List

Load a list of position to be reached sequentially

The list is an ASCII file containing one comment line and then a list of number pairs separated by a space corresponing to the position in mm to be reached and the time in seconds to wait at this position. Examples can be found in the program directory C:\Documents and Settings\ntofdaq\Desktop\Motorcontrol_nELBE

Ref

Find absolute zero position

The motor will move the detector until the limit switch is triggered and slowly move back about 3.5 mm.

Lim+

Move 5 mm in negative direction

This button has to be used, when the limit switch was triggered before the program was started.

Lim-

Move 5 mm in positive direction

This button has to be used, when the limit switch was triggered before the program was started.

stop motor

Immediately stop current movement

If the List option is in use and the detector is waiting at a certain position this button will come into effect not before the waiting time has passed.

quit program

Stop the program regularly

kill

Stop the program irregularly

current speed/position

Current velocity and absolute position of the detector

reference/initalized

Displays that lights up green if the motor is inialized and/or referenced

How to run a beam scan

To perform a beam scan the transmission DAQ has to be used and the following preparations have to be done (if not yet done):

tell the operators that you will do some test measurement, so that they should not change the beam settings and call you in case of a beam failure

move scanner to beam position given in Tab.1 (velocity = 250 mm/min) and adjust voltage (cf. Tab. 2) to get count rate of about 10-20 kHz

move both scanner to zero position

position = 0 mm, velocity = 250 mm/min

Two different kinds of beam scans are usually performed. A wide range scan with moderate velocity over the the complete distance the detector can move or a narrow range scan with lowest velosity over a distance just covering the beam. The former takes 34 minutes the latter 28 minuntes.